1. Field of the Invention
This invention relates to surgical modifications to the eye. In a specific embodiment, the invention provides ophthalmic surgery techniques which employ a laser to effect ablative photodecomposition of corneal tissue to correct presbyopia and/or other vision defects.
With aging, a condition of the eye known as presbyopia develops. With this condition, the crystalline lens of the eye loses the ability to focus on near objects when the eye is corrected for far-vision.
Presbyopia is often treated with bifocal eyeglasses. With bifocals, one portion of the lens is corrected for far-vision, and another portion of the lens is corrected for near-vision. By looking down through the bifocals, the user looks through the portion of the lens corrected for near-vision. When viewing distant objects, the user looks higher, through the portion of the bifocals corrected for far-vision.
Efforts have been made to treat presbyopia using partitioned lenses positioned directly over the pupil of the eye. Examples include multifocal contact lenses. Unfortunately, when presbyopia is corrected with bifocal or multifocal lenses attached to the cornea, the user is simultaneously looking through the near- and far-vision corrected lenses. As a result, the user will see both in-focus and out-of-focus images simultaneously when viewing an object. This out-of-focus image superimposed on the in-focus image can cause glare and degrade vision when viewing objects at low contrast.
Another technique for treating presbyopia has been to correct one eye of the patient for near-vision and to correct the other eye for distance-vision. This technique is known as monovision. With monovision, a patient uses one eye to see distant objects and the other eye to see close objects. Unfortunately with monovision, the patient may not clearly see objects that are intermediately positioned because the object is out-of-focus for both eyes. Also, a patient may have trouble seeing with only one eye.
Laser-based systems and methods are known for enabling ophthalmic surgery on the cornea in order to correct vision defects by the technique known as ablative photodecomposition. Changing the shape of the anterior surface of the cornea will change the optical properties of an eye. These ablative photodecomposition systems and methods control ultraviolet laser radiation flux density and exposure time upon the cornea so as to achieve a desired surface change in the cornea and thereby correct an optical defect.
Several different ablative photodecomposition techniques have been described to correct specific optical errors of the eye. For example, a myopic condition may be corrected by laser sculpting a corneal surface to reduce curvature. An astigmatic condition, which is typically characterized by a cylindrical component of curvature (departing from the otherwise generally spherical curvature of the cornea), can be corrected by a cylindrical ablation. Laser sculpting a corneal surface to increase the curvature can correct a hyperopic condition.
In a typical laser surgical procedure, the optically functional region of the corneal surface to be ablated is designated the optical zone. Depending on the nature of the desired optical correction, the optical zone may or may not be centered on the center of the pupil or on the apex of the anterior corneal surface. One technique for increasing the curvature of the optical zone for hyperopia error correction involves selectively varying the area of the cornea exposed to the laser beam radiation so as to produce an essentially spherical surface profile of increased curvature. This selective variation of the irradiated area may be accomplished in a variety of ways. For example, the optical zone can be scanned with a laser beam having a relatively small cross-sectional area (compared to the optical zone) in such a manner that the ablation depth increases with distance from the intended center of ablation. The result is a substantially spherical profile for the anterior corneal surface with maximum depth of cut at the extreme outer boundary of the optical zone. Another technique for sculpting the optical zone employs a rotatable mask having a plurality of apertures. The apertures are sequentially introduced into the laser beam path to provide progressive shaping of the laser beam in order to achieve the desired profile.
Efforts have also been made to treat presbyopia using ablative photodecomposition. One specific technique of treating presbyopia creates near-vision correction by ablating a region of the lower portion of the cornea adjacent the pupil rim. With this eccentric positioning of the ablation, the near-vision lens is not centered over the pupil. Consequently, constriction of the pupil may occlude the ablated near-vision lens. Constriction of the pupil is a natural response of the eye to illumination, and could potentially disrupt near-vision.
Alternative suggested presbyopia treatments include laser ablation of a small annular region of the cornea (having a diameter not exceeding 3.5 mm), or the ablation of a central lens for near-vision, surrounded by a gradual blend zone, and then a peripheral far-vision lens, all within the optically used portion of the cornea.
Efforts have been made in the past to laser sculpt a transition zone to provide a more gradual sloping of the walls and to eliminate the sharp discontinuity between the ablation zone and the surrounding untreated cornea. These efforts have included the use of a beam rotation or scanning mechanism operated by a computer to provide programmed ablation of the transition zone to achieve a sigmoid or other profile. While somewhat effective, these efforts often suffer from the added complexity of additional optical elements, such as a rotatable off-axis mirror or revolving prism having suitable optical properties.
2. Description of the Background Art
Systems and methods relevant to laser-based treatments for presbyopia are disclosed in the following U.S. patents and patent applications, the entire disclosures of which are hereby incorporated by reference: U.S. Pat. No. 5,395,356, issued Mar. 7, 1995, for "Correction of Presbyopia by Photorefractive Keratectomy"; U.S. Pat. No. 5,533,997, issued Jul. 9, 1996, "Apparatus and Method for Performing Presbyopia Correction"; and U.S. Pat. No. 5,314,422, issued May 24, 1994, for "Equipment for the Correction of Presbyopia by Remodeling the Corneal Surface by Means of Photoablation".
Ablative photodecomposition systems and methods are disclosed in the following U.S. patents and patent applications, the entire disclosures of which are hereby incorporated by reference: U.S. Pat. No. 4,665,913, issued May 19, 1987, for "Method for Ophthalmical Surgery"; U.S. Pat. No. 4,669,466, issued Jun. 2, 1987, for "Method and Apparatus for Analysis and Correction of Abnormal Refractive Errors of the Eye"; U.S. Pat. No. 4,732,148, issued Mar. 22, 1988, for "Method for Performing Ophthalmic Laser Surgery"; U.S. Pat. No. 4,770,172, issued Sep. 13, 1988, for "Method of Laser Sculpture of the Optically Used Portion of the Cornea"; U.S. Pat. No. 4,773,414, issued Sep. 27, 1988, for "Method of Laser Sculpture of the Optically Used Portion of the Cornea"; U.S. patent application Ser. No. 07/109,812, filed Oct. 16, 1987, for "Laser Surgery Method and Apparatus"; U.S. Pat. No. 5,163,934, issued Nov. 17, 1992, for "Photorefractive Keratectomy"; U.S. Pat. No. 5,556,395, issued Sep. 17, 1996, for "Method and System for Laser Treatment of Refractive Error Using an Offset Image of a Rotatable Mask"; U.S. patent application Ser. No. 08/368,799, filed Jan. 4, 1995, for "Method and Apparatus for Temporal and Spatial Beam Integration"; U.S. patent application Ser. No. 08/058,599, filed May 7, 1993, for "Method and System for Laser Treatment of Refractive Errors Using Offset Imaging"; U.S. Pat. No. 5,683,379, issued Nov. 4, 1997, for "Apparatus for Modifying the Surface of the Eye Through Large Beam Laser Polishing and Method of Controlling the Apparatus"; and U.S. Pat. No. 5,827,264, issued Oct. 27, 1998 for "Method of Controlling Apparatus for Modifying the Surface of the Eye Through Large Beam Laser Polishing."
Techniques for treating presbyopia with contact lenses are disclosed in the following U.S. patents and patent applications, the entire disclosures of which are hereby incorporated by reference: U.S. Pat. No. 5,835,192, issued Nov. 10, 1998, for "Contact Lens and Method of Fitting a Contact Lens"; U.S. Pat. No. 5,485,228 issued Jan. 16, 1996 for "Multifocal Ophthalmic Lens Pair;" and U.S. Pat. No. 5,864,379 issued Jan. 26, 1999 for "Contact Lens and Process for Fitting."